You probably run the thing as a USB 1 device. Make sure that the ehci_hcd module is loaded. You can see the speed in /proc/bus/usb/devices, where Spd=12 means usb1 and Spd=480 means usb2 high speed. In practice, i never get 40MB/s from my usb2 hdd. it usually varies between 10 and 25MB/s. Cheers, Volker